Rain Gardens – Land- Scaped Areas Planted to Wild flowers and Other Native Vegetation That Soak up Rain Water, Mainly from the Roof of a House Or Other Building
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A how-to manual for homeowners Your personal contribution to cleaner water Homeowners in many parts of the country are catching on to rain gardens – land- scaped areas planted to wild flowers and other native vegetation that soak up rain water, mainly from the roof of a house or other building. The rain garden fills with a few inches of water after a storm and the water slowly filters into the ground rather than running off to a storm drain. Compared to a conventional patch of lawn, a rain garden allows about 30% more water to soak into the ground. Why are rain gardens important? As cities and suburbs grow and replace forests and agricultural land, increased stormwater runoff from impervious surfaces becomes a problem. Stormwater runoff from developed areas increases flooding; carries pollutants from streets, parking lots and even lawns into local streams and lakes; and leads to costly municipal improvements in stormwater treatment structures. By reducing stormwater runoff, rain gardens can be a valuable part of changing these trends. While an individual rain garden may seem like a small thing, collectively they produce substantial neighborhood and community environmental benefits. Rain gardens work for us in several ways: g Increasing the amount of water that filters into the ground, which recharges local and regional aquifers; g Helping protect communities from flooding and drainage problems; g Helping protect streams and lakes from pollutants carried by urban stormwater – lawn fertilizers and pesticides, oil and other fluids that leak from cars, and numerous harmful substances that wash off roofs and paved areas; g Enhancing the beauty of yards and neighborhoods; g Providing valuable habitat for birds, butterflies and many beneficial insects. 2 Who should use this manual? This manual provides homeowners and landscape professionals with the Frequently asked questions information needed to design and build rain Does a rain garden form a pond? gardens on residential lots. Guidelines presented in this No. The rain water will soak in so the rain garden is dry manual can also be used to between rainfalls. (Note: some rain gardens can be treat roof runoff at com- designed to include a permanent pond, but that type of mercial and institutional sites. However, the manual rain garden is not addressed in this publication). should not be used to design rain gardens for Are they a breeding ground for mosquitoes? parking lots, busy streets and other heavily used No. Mosquitoes need 7 to 12 days to lay and hatch eggs, paved areas where and standing water in the rain garden will last for a few stormwater would require pretreatment before hours after most storms. Mosquitoes are more likely to lay entering a rain garden. eggs in bird baths, storm sewers, and lawns than in a sunny rain garden. Also rain gardens attract dragonflies, which eat mosquitoes! Do they require a lot of maintenance? Rain gardens can be maintained with little effort after the plants are established. Some weeding and watering will be needed in the first two years, and perhaps some thinning in later years as the plants mature. Is a rain garden expensive? It doesn’t have to be. A family and a few friends can provide the labor. The main cost will be purchasing the plants, and even this cost can be minimized by using some native plants that might already exist in the yard or in a neighbor’s yard. Rain Gardens – A how-to manual for homeowners 3 • • • • • Step Sizing and Siting the Rain Garden This section of the manual covers rain gar- den basics – where to put the rain garden, how big to make it, how deep to dig it, and what kind of soils and slope are best. Following the instructions in this section is the best way to ensure a successful rain garden project. An extension of PVC pipe helps direct downspout water If you already know the size you want your rain to this rain garden. garden to be, then skip ahead to the section about building the rain garden. However, take time read the pointers about location, and do find the slope of the lawn. If the location has a slope more than about 12%, it’s best to pick a different location because of the effort it will take to create a level rain garden. Where should the rain garden go? Home rain gardens can be in one of two places – near the house to catch only roof runoff or farther out on the lawn to collect water from the lawn and roof. (Figure 1 shows the possible locations on a residential lot.) To help decide where to put a rain garden, consider these points: • The rain garden should be at least 10 feet from the house so infiltrating water doesn’t seep into the foundation. • Do not place the rain garden directly over a septic system. • It may be tempting to put the rain garden in a part of the yard where water already ponds. Don’t! The goal of a rain garden is to encourage infiltration, and your yard’s wet patches show where infiltration is slow. • It is better to build the rain garden in full or partial sun, not directly under a big tree. • Putting the rain garden in a flatter part of the yard will make digging much easier. For example, a rain garden 10 feet wide on a 10% slope must be 12 inches deep to be level, unless you import topsoil or use cut and fill. 4 Consider your overall landscape When considering placement of your rain throughout the landscape. Determine how garden, design with the end in mind. far or how close you want your rain garden Carefully consider how the rain garden can to outdoor gathering spaces or other play be integrated into existing and future areas. Why not locate it near a patio where landscaping. Also, pay attention to views you can take advantage of the colors and from inside the house as well as those fragrances for hours on end! Figure 1 A rain garden can rain garden be built in the front or back length yard. Pick a pleasing shape for the rain garden. Crescent, rain kidney, and teardrop shapes garden seem to work well. width not within 10' > 30' of foundation from down spout roof and lawn drainage area to back rain garden roof drainage area to front rain garden close to down spout rain garden width rain garden length street Rain Gardens – A how-to manual for homeowners 5 > 10' Figure 2 Rain gardens should from be located at least 10 feet from foundation the house, on a gentle slope that catches downspout water. > 30' close from to berm berm down spout down spout < 12% street slope How big should the rain garden be? The surface area of the rain garden can be almost any size, but time and cost will always be important con- siderations in sizing decisions. Any reasonably sized rain garden will provide some stormwater runoff control. A typical residential rain garden ranges from 100 to 300 square feet. Rain gardens can be smaller than 100 square feet, but very small gardens have little plant variety. If a rain garden is larger than 300 square feet it takes a lot more time to dig, is more difficult to make level, and could be hard on your budget. The size of the rain garden will depend on •how deep the garden will be, • what type of soils the garden will be planted in, and •how much roof and/or lawn will drain to the garden. Guidelines are not rules… This information, along with the sizing factor from the tables on The sizing guidelines page 9, will determine the surface area of the rain garden. described in this manual are based on a goal of controlling 100% of the runoff for the Digging with a rented backhoe. average rainfall year while keeping the size of the rain garden reasonable. Establish- ing a 100% runoff goal helps compensate for some of the errors that creep into the design and construction of any rain garden. If you follow the guidelines in the manual and decide the calculated surface area is just too large for your goals, it is perfectly acceptable to make the rain garden smaller. The rain garden can be up to 30% smaller and still control almost 90% of the annual runoff. On the other hand, it is fine to make the rain garden bigger than the guidelines indicate. 6 How Deep Should the Rain Garden Be? A typical rain garden is between four and eight inches deep. A rain garden more than eight inches deep might pond water too long, look like a hole in the ground, and present a tripping hazard for somebody stepping into it. A rain garden much less than four inches deep will need an excessive amount of surface area to provide enough water storage to infiltrate the larger storms. No matter what the depth of downhill the string must be level uphill the rain garden, the goal is to stake stake keep the garden level. Digging a very shallow rain garden on a width steep lawn will require bringing in extra topsoil to bring the height downslope part of the garden up to the same height as the The string should be tied to up-slope part of the garden. As Figure 3 the base of the uphill stake, then tied to the slope gets steeper, it is easi- the downhill stake at the same level. er to dig the rain garden a little deeper to make it level.